1. Field of the Invention
This invention relates to the characterization of bipolar transistors, and more particularly to a novel heterojunction bipolar transistor (HBT) test structure and method in which test probe pads are provided lateral to respective transistor regions prior to forming an upper metallization.
2. Description of the Related Art
In characterizing newly manufactured bipolar transistors to determine whether they operate properly and meet specifications, a small portion of the transistors on a wafer (typically less than one percent) are dedicated as test devices. Since all of the transistors on the wafer are fabricated in a common process, they will generally exhibit common operating characteristics. Therefore, characterizing the relatively few test transistors can be taken as a characterization of the larger number of circuit transistors that are employed in the operating circuitry of the wafer.
When the transistor fabrication has been completed, the wafer is coated with one or more dielectric layers, with metallized leads formed on each layer and extending through the underlying dielectric to establish electrical connections with the transistors. The metallization for the test transistors is provided with enlarged probe pads on the upper dielectric surface so that the test transistors can be electrically accessed via test probes brought into contact with the contact pads. (The term “metallization” as used herein is not limited to conventional metal elements, but also encompasses alternate conductive mechanisms such as heavily doped semiconductor.)
While this approach has been found to be effective in characterizing a large number of transistors by actually testing only a small portion, it requires that the wafer fabrication be fully completed before characterization can take place. Thus, if the test transistors do not meet specifications, the completed wafer must be discarded. This is costly in terms of both processing time and expense.